1. Field of the Invention
The present invention relates to power amplifiers and, more particularly, to microwave power amplifiers employing microwave integrated circuit fabrication techniques.
2. Description of the Prior Art
In the pertinent art, semiconductor devices that operate on a signal to modify the signal characteristics in a selectively controlled manner are generally referred to as active semiconductor devices. Examples are field-effect transistors (FET) and insulated-gate field-effect transistors (IGFET). Active semiconductor devices with relatively high power capabilities have generally been constructed by merely paralleling a number of cells in the active device. However, paralleling cells in a single device results in fundamental disadvantages and limitations in the device operating parameters, such as efficiency and input/output impedance. These disadvantages limit the number of device cells that can be used together and, therefore, the total power amplification that can be achieved.
As used herein, elemental amplifiers are specifically defined to be an active device together with impedance matching networks and/or stabilization networks associated therewith. As also used herein, composite amplifiers are specifically defined to be an array or group of separate active devices or elemental amplifiers that are electrically interconnected to form an amplifying circuit. In the past, composite amplifiers have employed a plurality of active devices or elemental amplifiers in various circuit combinations to construct amplifier circuits with high power capacity.
Generally, composite amplifiers have been found to have more satisfactory performance than single active devices or single elemental amplifiers because more active devices or elemental amplifiers can be combined in the circuit of a composite amplifier than the number of cells that can be parallel constructed in a single active device. It has also been found that the composite amplifier is less sensitive to variations between elemental amplifier parameters than when the device cells are combined directly. Another advantage in combining the elemental amplifiers in a composite amplifier is that isolation between elemental amplifiers can be provided when they are combined on a circuit level as in composite amplifiers.
Furthermore, the output power tends to be affected in a predictable manner upon failure of one or more of the elemental amplifiers whereas the failure of one or more parallel cells tends to cause unpredictable changes in the output power for the device.
In the prior art, composite power amplifiers which have generally operated at L and S-band frequencies, have generally employed bipolar power devices. However, at X-band, techniques developed for bipolar amplifiers are too inefficient or have too narrow a bandwidth to have useful application. For example, the power amplifier comprising a binary tree of quadrature or in-phase hybrids has been found to have high isolation between transistors over a wide bandwidth, but is too inefficient at X-band to be used to combine more than four devices. Furthermore, the binary nature of that amplifier circuit requires that 2.sup.n devices (where n is an integer) must be combined to avoid wasting power.
Combining techniques for power amplifiers that have adequate efficiency at X-band have been developed. However, the bandwidth of these amplifiers has been found to be unacceptably narrow. For example, a resonant cavity power combiner is known which combines power from two terminal elemental amplifiers at X-band frequencies with high efficiency. However, the high-Q of the resonant cavity power combiner makes the composite amplifier inherently narrow band.
Another significant limitation of prior combiner circuits is that they have generally been unable to take advantage of the lower production costs and other benefits of planar metallization technology. For example, planar metallization technology has not generally been applied to the N-way Wilkinson combiner because of topological problems that arise in physically locating the isolating resistors so that they can be conveniently assembled but yet can properly dissipate incident power upon failure of the elemental amplifiers. Inadequate capacity of the isolating resistors to dissipate power causes unpredictable effects in the power output level of the composite amplifier upon failure of an elemental amplifier.
Accordingly, there was a need in the prior art for a composite power amplifier that was efficient that had wide bandwidth at X-band frequencies, and that could be produced in accordance with planar metallization technology.